1. Field of the Invention
The present invention relates to a soldering method. More specifically, the present invention relates to a method of electrically and mechanically connecting a substrate such as a rigid substrate and a flexible substrate, or a method of electrically and mechanically mounting on a substrate a connected member such as an IC (integrated circuit) chip, a discrete element and so on.
2. Related Arts
In recent years, there have been growing demands, in the installation of electronic products, for techniques of connecting and mounting components which are to be put in place at microscopic intervals. In the past, flux has been used in the soldering (which hereinafter implies an electrical connection by means of soldering and brazing). The flux has to be removed by cleaning after a connecting operation is finished since it is corrosive. In view of the environmental problems, however, there has been increasing restrictions imposed on the use of cleaning agents which contain CFCs (chlorofluorocarbons). A cleaning operation is sometimes difficult to be performed in the case of a soldering operation which has to be executed inside a package. Consequently, attempts have been made to implement soldering operations dispensing with cleaning operations by using low active flux which does not require the cleaning operation, or to perform a soldering operation in an inactive atmosphere without using flux if it has to be executed in a location which does not allow a cleaning operation. In the cases like these where low active flux is used or soldering operations are performed without using flux, it is difficult to secure a sufficient area for connection due to the effect of an oxide film on the surface of solder, which has given rise to such problems as the soldering being insufficient in strength and the connection being low in reliability, or voids occurring inside solder.
As an example of a soldering which will solve such problems as mentioned above, there is disclosed in Japanese Patent Laid Open No. 6-226485 a method in which a soldering is performed with a solder alloy coated with paraffin wax on its surface. Paying attention to the fact that the voids are caused by an oxide film, the technique has employed a method of covering solder with paraffin wax so as to prevent an oxide film from being grown on the surface of the solder.
Although the above-mentioned technique shows a method for preventing an oxide film from being formed, and for keeping and using solder under a condition in which there is no penetration of water, but it does not deal with soldering which has to be performed under a condition in which there is an oxide film formed on solder. In other words, soldering cannot be executed unless terminals have been solder-plated in advance as in the case of connections between terminals, and in the event of solder surface being oxidized, solder coated with paraffine wax cannot be used for connections between terminals.